Identification of a phenotype-specific enhancer in the first intron of the rat collagen II gene.

The regulation of the collagen II gene was investigated by transfecting plasmids containing potential regulatory sequences of this gene coupled to the gene for chloramphenicol acetyltransferase (CAT) into various cells. The 5' flanking region of this gene functioned as a weak promoter when transfected into chicken chondrocytes or fibroblasts. Inclusion of an 800-base fragment from the first intron, however, increased transcription of CAT approximately 18-fold in chicken chondrocytes and in differentiating limb bud cells but not in fibroblasts, myoblasts, muscle-derived fibroblasts, or teratocarcinoma cells. Furthermore, this fragment was active when placed either upstream or downstream of the CAT gene and when present in either orientation. The activity of this enhancer was examined in relation to differentiation by using "micromass" culture of limb bud mesenchyme cells undergoing chondrogenesis. In this system, no response to the enhancer was observed in undifferentiated limb bud mesenchyme cells. Following differentiation into chondrocytes, a 13-fold enhancer effect was observed in these cells. Finally, all-trans-retinoic acid, a known teratogen, dramatically suppressed enhancer activity in chondrocytes and differentiating limb bud mesenchyme cells. These results suggest that the collagen II gene contains an enhancer element in the first intron that is involved in cell-specific expression.